1. Field of the Invention
The present invention relates to an image forming apparatus including a contact transfer member which contacts an image bearing member, so as to transfer a toner image on the image bearing member to a transfer material. In particular, it relates to an image forming apparatus capable of changing the condition of the transfer member in response to the type of the transfer material.
2. Description of the Related Art
A contact transfer member, which contacts an image bearing member so as to transfer a toner image on the image bearing member to a transfer material, has an advantage that the conveyance of the transfer material during transfer is stabilized and has been used widely.
However, in the case where the contact transfer member is used, the following problems occur.
The optimum relationship between a transfer voltage applied to the contact transfer member and a transfer current passing the contact transfer member when the transfer voltage is applied varies depending on the type of transfer material. Therefore, appropriate transfer may not be performed depending on the type of transfer material.
That is, in the case where the length in a direction orthogonal to the conveyance direction (hereafter referred to as “width direction”) of a transfer material to be used is short, the transfer current concentrates on a non-paper-passing portion of a transfer portion, the transfer current passing a paper-passing portion becomes insufficient, and a toner image is not satisfactorily transferred. In the case where cardboard is used, as shown in FIG. 9, when a part of the front-end portion of the transfer material enters the transfer portion, the transfer current concentrates on a portion, in which no transfer material is present, of a transfer portion, the transfer current passing the transfer material becomes insufficient, and a toner image is not satisfactorily transferred to the front-end portion. In these cases, it is better that the resistance of the contact transfer member becomes higher.
On the other hand, in the case where the length in the width direction of the transfer material to be used is long, the problem hardly occurs in that the transfer current passing the transfer material becomes insufficient due to concentration of the transfer current on the non-paper-passing portion. In this case, since the voltage applied to the contact transfer member during the transfer can be decreased, it is better that the resistance of the contact transfer member becomes lower.
For the configuration to overcome the above-described problems, Japanese Patent Laid-Open No. 5-281859 discloses a transfer belt which contacts an image bearing member and which is provided with a high resistance portion and a low resistance portion in the direction of rotation.
However, if the configuration, in which the high resistance portion and the low resistance portion are disposed in the contact transfer member and are switched in response to the type of the transfer material, is used, there is a problem in that the image forming apparatus is upsized.